Needle-free injection systems provide an alternative to standard fluid delivery systems, which generally use a needle adapted to penetrate the outer surface of a target. Typically, needle-free injection systems are designed to eject the fluid from a fluid chamber with sufficient pressure to allow the fluid to penetrate the target to the desired degree. For example, common applications for needle-free injection systems include delivering intradermal, subcutaneous, and intramuscular injections into or through a recipient's skin. For each of these applications, the fluid must be ejected from the system with sufficient pressure to allow the fluid to penetrate the tough exterior dermal layers of the recipient's skin.
Intradermal injections, the least invasive of the three types, typically are employed when the injectate dose is very small or when visualization of the subject's local response to the injectate is desired. Subcutaneous injections typically are employed when prolonging the absorption time of the medication is desirable, when the dose is relatively small, or when the injectate is non-irritating. Intramuscular injections, the most invasive of the three types, typically are employed when rapid absorption is desired, when the medication is irritating, or when the dose is relatively large.
The efficacy of delivering a needle-free injection to a desired layer of the skin, subcutaneous tissues, or muscle may depend not only on the pressure with which fluid is ejected from the device, but also on the distance of the spray nozzle from the skin. For instance, whereas intramuscular injections typically require the nozzle to be placed directly against the skin to achieve the desired depth of penetration, intradermal injections typically require the use of a spacer between the nozzle and the skin surface. Additionally, the propulsion mechanism employed in many needle-free injector systems involves compressed gas. Such systems require the compressed gas to be replenished when depleted.